Such housed electronic components (packages) are known, for example, as chip size packages, as multichip modules, or as BGA components. They essentially comprise one or more semiconductor chips, which are mechanically mounted and electrically contact-connected on a carrier substrate. For this purpose, a semiconductor chip is located on one side of the carrier substrate. Through-plating leads extend through the carrier substrate as far as its second side. Electrical external contacts on the second side of the carrier substrate are used contact the package to a circuit board. Conductor tracks are furthermore arranged on the second or, if necessary, on both sides of the carrier substrate, by means of which conductor tracks the various contact areas are connected to one another in such a way that the external contacts are electrically contact-connected to the corresponding contacts of the semiconductor chip.
The through-plating may be embodied in various ways according to the embodiment of the electrical contacts of the semiconductor chip and the mechanical mounting thereof on the carrier substrate. By way of example, in the case of central contact rows and in the case where the semiconductor chip is mounted with the active side downward (face-down), the carrier substrate has a single, relatively large passage (bonding channel) in the central region of the carrier substrate. The through-plating is equally possible by means of a relatively large number of areally distributed through contacts, in the form of metallized holes in the carrier substrate.
In order to protect the semiconductor chip against ambient influences and against damage during the further processing of the package, the semiconductor chip is surrounded by a housing. This is effected, for example, by means of a potting composition in an injection molding method, so that the entire electronic component with the exception of the electrical external contacts is sealed by a plastic composition.
An ingress of moisture, dependent on the respective materials, into the package is also always associated with the various material components of such a package, the carrier substrate, the housing material and, if appropriate, the adhesive material for fixing the semiconductor chip on the carrier substrate. Moreover, water vapor constantly diffuses through the housing into the interior thereof or small quantities of water penetrate along the connecting areas between the housing and the carrier substrate. Both the residual moisture that has remained in the package and the water vapor that constantly penetrates from outside under a normal atmosphere lead to disturbances or else to the failure of the package, particularly if the package is exposed to elevated temperatures as is effected for example during the soldering process for the integration of the package. On account of the water content in the package and the elevated temperature, vapor pressure is developed in the housing if the water vapor cannot emerge from the housing in a timely manner. The internal pressure, which thus builds up, can lead to the destruction of the housing and consequently of the package as a result of the known popcorn effect, and to delamination along the contact area of the housing with the carrier substrate.
In order to assess the residual moisture present in the package and the associated possible storage of the package under a normal atmosphere, which means without particular measures for keeping it dry such as, for example, without dry packaging, the packages are subjected to various loading tests and, with conclusions being drawn therefrom, are assigned to corresponding classifications, the so-called moisture sensitivity levels. Particularly during soldering processes using lead-free solder, which proceed at relatively high temperatures, it is necessary to use packages having the highest classification level, MSL1/260° C. These packages have to withstand, without damage, being artificially stored for several days at 85° C. and 85% relative air humidity and a subsequent soldering simulation at 260° C.
In order to reduce the water content, special materials are used for mounting and housing. Thus, by way of example, a potting composition for producing the housing with a relatively high proportion of filler is used since the water absorption of the potting composition can thus be significantly reduced. German Patent Application No. 197 41 437 A1 describes the use of a potting composition whose basic epoxy resin is based on a biphenyl structure so that the potting composition is distinguished by low water absorption besides a high proportion of filler. In order to reduce delamination, moreover, the semiconductor chip is provided with a buffer layer made of polybenzoxazole on its side facing the potting composition, which buffer layer is intended to bring about a good adhesion imparted between the materials of the chip and the potting composition and thus the reduction of the ingress of water in this region.
However, these measures relate only to the potting composition and the contact thereof with respect to the semiconductor chip and cannot be applied to the connection of both materials to a carrier substrate or to other housing embodiments. This is because, particularly when glass-fiber-reinforced epoxy resin is used, capillary acting moisture paths arise along the glass fibers, and they cannot be prevented by selected mounting materials either. These moisture paths have particularly disadvantageous effects if the semiconductor chip is adhesively bonded on such a carrier substrate since the adhesive material has a high proportion of water-absorbing polymer that cannot be reduced by a filler without impairing the adhesive properties.
If the carrier substrate itself also has a specific water content, which may occur in the case of plastic substrates, the carrier substrate also participates in the ingress of moisture into the package and is affected by the damaging popcorn effect and delamination.
On account of the changing number of material components involved in the package, their further requirements that principally have to be satisfied with regard to their electrical, mechanical and also thermal properties and their compatibility with one another, an initial ingress of moisture is practically unavoidable. Subsequent drying, just like preventing the ingress of water or water vapor, is also associated with an increased outlay in respect of time and materials and is, therefore, not a practicable solution particularly on account of rising production numbers.